Acrylic artificial marble prepared by compounding inorganic fillers such as aluminum hydroxide and the like into an acrylic resin has various prominent functions and properties such as excellent mold appearance, soft feeling, weatherability and the like, and is widely used for counters such as a kitchen counter and the like, washing and dressing stands, waterproof pan, an other architectural uses. These are usually produced by a casting method in which a so-called premix prepared by dispersing inorganic fillers into an acrylic syrup is filled into a mold, and this is cured and polymerized at relatively lower temperature. However, since this acrylic syrup has a lower boiling point, the curing temperature has to be lowered and consequently the molding time becomes longer, therefore, productivity decreases. Further, since the filling property of the premix into a mold is not successful, the shape of a molded article is restricted.
For improving these defects, there has been conventionally effected investigation for producing acrylic artificial marble by heat and compression molding of BMC obtained by thickening a resin syrup with a thickening agent.
For example, Japanese Patent Application Laid-Open (JP-A) No. 5-32720 discloses an acrylic BMC for artificial marble excellent in handling property and molding property, prepared by compounding inorganic fillers and a crosslinked resin power having specific degree of swelling obtained by suspension polymerization into an acrylic syrup obtained by previously dissolving an acrylic polymer in an acrylic monomer.
Further, JP-A No. 6-298883 discloses an acrylic BMC for artificial marble excellent in low constriction property in heat-curing, prepared by compounding organic fillers and a thermoplastic acrylic resin powder into an acrylic syrup obtained by previously dissolving an acrylic polymer in an acrylic monomer, the thermoplastic acrylic resin powder being poor soluble in the syrup.
Further, JP-A No. 6-313019 discloses an acrylic BMC for artificial marble of which cracking in molding is prevented and appearance of the resulting molded article and thickening stability thereof are improved, prepared by compounding a resin powder obtained by spray-drying treatment of a crosslinked polymer obtained by emulsion polymerization into an acrylic syrup obtained by previously dissolving an acrylic polymer in an acrylic monomer.
Further, Japanese Patent Application Publication (JP-B) No. 5-13899 discloses an acrylic BMC for artificial marble of excellent in heat resistance, prepared by compounding inorganic fillers and an acrylic resin powder having a specific molecular weight into an acrylic monomer containing a specific amount of a crosslinking agent.
However, in the methods disclosed in JP-A Nos. 5-32720, 6-298883 and 6-313019, since an acrylic polymer is dissolved in an acrylic monomer to prepare a syrup, number of processes is large and production cost tends to increase. Further, when resin powders disclosed in these publications are used as thickening agents, there is a tendency that aging of a premix for obtaining an acrylic BMC requires a long period of time (about 24 hours) and productivity decreases.
On the other hand, in the method disclosed in JP-B No. 5-13899, an acrylic BMC is obtained directly by mixing an acrylic monomer, inorganic filler and polymer powder not via an acrylic syrup. However, since the mixture is kneaded at a temperature of 60.degree. C. for 30 minutes or more for dissolving the polymer powder and thickening the mixture, long period heat history is required for obtaining the acrylic BMC and storage stability of the resulting acrylic BMC deteriorates. Further, when a grain-like acrylic BMC is going to be obtained by compounding a colored resin particle in this method, the colored resin particle is dissolved during kneading and grain pattern becomes extremely unclear or grain pattern disappears, leading to poor appearance.
To solve these problems, the present inventors have found that when a polymer powder having certain oil absorption, certain degree of swelling and certain specific surface area is used as a thickening agent, there is obtained an acrylic BMC which exhibits high thickening speed, requires no aging and provides clear grain pattern (International Publication WO97/40098).